1. Field of the Invention
The present invention relates to a method for preparing 3-(hydroxyphenylphosphinyl)-propanoic acid. 3 -(hydroxyphenylphosphinyl)-propanoic acid, which is commonly used as a flame retardant for polyester resins, is generally prepared by the condensation of phenylphosphonous dichloride and acrylic acid, followed by the hydrolysis of the condensate, 3-(chlorophenylphosphinyl)-propionylchloride, as shown in the following reaction formula I: ##STR1##
2. Description of the Prior Art
Pudovik et al. reported the above procedure in the Russian Journal of Organic Chemistry (Vol. 137, pp. 423-427) in 1967. Since then, there have been disclosed a number of improved procedures which could increase the production yield by controlling a reactant mole fraction or reaction temperature. For example, U.S. Pat. No. 4,081,463 (Birum et al.) proposes a method in which acrylic acid was used at an amount more by 25.about.45 mol % (more preferably 30.about.40 mol % ) than that of phenylphosphonous dichloride, insisting that the production yield can be increased by using excess acrylic acid because a portion of phenylphosphonous dichloride remains unreacted when these reactants are present at the same equivalent. However, this method is problematic in that not only the reactant is wasted due to the excess of acrylic acid, but also the production yield is not increased to the is satisfactory extent. Another reference directed to the preparation of 3-(hydroxyphenylphosphinyl)-propanoic acid is found in U.S. Pat. No. 4,769,182, yielded to Hazen. It discloses that the object compound can be produced at high production yields by using acrylic acid at a relatively small amount, but greater by 5.about.10 mol % than that of phenylphosphonous dichloride and maintaining the reaction temperature at 115.about.150 .degree. C. during the reaction of the reactants. In the above-cited patent, Hazen insisted that acrylic acid was not consumed nor converted into an unreactable material, but converted into certain acrylic acid derivatives which were then reacted with remaining phenylphosphonous dichloride at the elevated reaction temperature, which led to an improvement in the production yield. However, despite the enhanced production yield, this method suffers from a weakness in that the effect of excess acrylic acid is abated because acrylic acid used in the reaction is self-condensed or boiled to vaporize owing to the high temperature.
Flame-retardant, 3-(hydroxyphenylphosphinyl)-propanoic acid-containing polyester is generally used for fibers such as curtains, coverlets and night dresses. One of the factors which have significant influence on the quality of these fiber products is color. The colors of the products are partly determined by the color of the polyester. Accordingly, the color of 3-(hydroxyphenylphosphinyl)-propanoic acid, which is added to provide flame retardation for polyester, is very important in determining the colors of final products.
As for 3-(hydroxyphenylphosphinyl)-propanoic acid prepared by the Hazen's method, it is slightly yellowish.